April, 2012
www.us-tech.com Getting PCBs Really Clean Continued from page 57
stood and obeyed, half of the cleaning is achieved. Consider demineralized water
which has a surface tension of about 80mN/m. The surface tension line bends as a function of an increase in temperature of the bath. The varia- tion is of about -10 mN/m. This is the reason why cleaners are warmed up in washing units — to reduce the sur- face tension and allow the liquid to move underneath cavities and com- ponents. There is the same issue with tap water, where its surface tension starts at 70mN/m at room temperature. When detergents are added to water, the surface tension of the medium drops down to 45-35 mN/m, according to the temperature of the bath. However, the big ques- tion remains, how can a “wetting product” be rinsed with some water which has a higher surface tension? For this reason many suppliers use additives or simple isopropanol, to better rinse, wet and dry parts. To get underneath the compo-
nents, glycols or formulated hydro- carbons are commonly used. Their surface tensions are lower than water and detergents, between 25-35 mN/m for the glycols and around 20mN/m for the formulated hydro- carbons. The same rinsing problem remains for these products with water, so it is preferable to use a selected final rinsing solvent which has a lower surface tension than these cleaners and which will finally dry the PCBs. As solvents evaporate and con- dense on the free-boards of the vapor
phase equipment, no residues are left on the PCBs and the surfaces, includ- ing under the components.
Mechanical Agitation There are many types of mechan-
ical agitation that can be used — sprays, sprays under immersion, ultrasonics, agitators, rotation, and others. Agitation provides an addition- al cleaning parameter which helps to penetrate, to dissolve and to unfasten
Cleaning process Aspersion, aqueous
Immersion US 40 kHz, aqueous Immersion jets, semi-aqueous
ticularly, quartz. The trials, which included 60 different assemblies con- taining quartz crystals — accordingly to Norm IPC-TM-650 — have demon- strated that none of the crystals have been affected or damaged in any way. The benefits of ultrasonics were easily determined. This cleaning study shows that
there are many parameters that affect cleaning efficiency. The final aspects and performances of these
Product Family Detergent 1 Detergent 2
Equipment type Dish washer type Sumps in line
Glycol formulation 3 Sumps in line
Immersion US 40 kHz, semi-aqueous Glycol formulation 3 Sumps in line Immersion US 30 kHz, mono-product Glycol formulation 4 Vacuum machine Immersion US 40 kHz, semi-aqueous Glycol formulation 5 Sumps in line Immersion, co-solvent mixed Immersion, co-solvent mixed
Form hydrocar/HFE Vapor degreaser Form.hydrocar70/HFE Vapor degreaser
Immersion US 25 kHz, co-solvent sep Form hydrocar/HFE Co-solvent/vapour degreaser Immersion US 38 kHz, co-solvent sep Form hydrocar /HFE Co-solvent/ vapour degreaser Immersion Jets, co-solvent separated Form hydrocar./HFE Co-solvent/vapour degreaser Immersion US 40 kHz, vapour phase Brominated solvent Vapour phase
Comparative results of different cleaning processes.
contaminants. This study evaluates all types of mechanical agitation and compares their efficacy. Many times, industrial PCB assemblers avoid using ultrasonics because of some fear about damaging the components, par-
PCBs are based on the mastering of cleaners, size of the assemblies, agi- tations and the cleaning processes. When the choice is based on solvency power, the lowest surface tension and the most efficient process, then a
Contact: Inventec Performance
Chemicals, 26 avenue du Petit Parc, 94683 Vincennes Cedex, France % 33 (0)1 43 98 75 00 fax: 33 (0)1 43 98 21 51 Web:
www.inventec.dehon.com r
perfect job can be reached matching the toughest specifications. The co- solvent/Vapor degreaser process with formulated hydrocarbon and HFE (hydrofluoroether), combined with ultrasonics or jets show the best per- formance. Contamination is below 0.2µg of Eq NaCl/cm2 for ranking 9 (on a scale of 1 to 10) and with a per- fect visual aspect under components. No damage of quartz could be found in any of the trials.
Comparative 1:10 best 4 1 3 7 7 1 4 2 9 9 8 4
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